This is the 190th most frequent Finnish word.
niistä
"Niistä" in Finnish means "of them" or "about them," and it can also mean "blow (one's nose)" depending on the context.
In this sentence, 'niistä' is in the elative case (indicating 'out of those') and refers to the movies previously mentioned. It conveys preference or selection from a set of items.
Pidin kaikista niistä elokuvista.
I liked all of those movies.
Here, 'niistä' also takes the elative case and is used to refer back to a previously mentioned topic or a group of items being the subject of discussion.
Keskustelimme niistä pitkään.
We discussed them for a long time.
In this example, 'niistä' works similarly, specifying a group from which something is selected, enjoyed, or evaluated.